1. Field of the Invention
The present invention relates to a manufacturing method for an electrooptic element using a ferroelectric material and an optical deflector including such an electrooptic element.
2. Description of the Prior Art
There are mainly two kinds of optical deflector: a mechanical deflector such as a galvanometer, a polygon mirror, or a micro electro mechanical system (MEMS) and a non-mechanical deflector such as an acoustic optical element or an electrooptic element. Among them, the electrooptic element is configured to control the traveling direction of light by electro-optic effect which is a change in the refractive index of a material in response to the application of an electric field. The change in the refractive index by the Pockels effect is expressed by the following formula:Δn∝rij×V/dwhere rij is an electro-optic constant (Pockels constant), V is an applied voltage, and d is an interval between electrodes applying a voltage.
The electrooptic element as an optical deflector is comprised of a ferroelectric made from a single crystal oxide material such as niobate lithium, lithium tantalite, titanate phosphate potassium, niobate potassium which are relatively cheap and stable at ambient temperature, and has a high phase transition temperature. Japanese Patent Application Publication No. S62-047627 discloses an optical deflector including a prism-shape electrode to apply a voltage to an electrooptic element so that it acquires an optical deflecting function. The principle of optical deflection is such that by an applied voltage, a difference in the reflective index of prism regions of the electrooptic element occurs by the Pockels effect, causing deflection of light propagating through the electrooptic element.
The electrooptic element has a disadvantage of a small deflection angle compared with the other kinds of optical deflectors. In view of increasing the deflection angle, a prism domain inversion optical element is proposed in a document by David A. Scrymgeour et al., Applied Optics, Vol. 40, No. 34 (December 2001), Japanese Examined Patent Application Publication No. H09-501245, and Japanese Patent Application Publication No. H09-146128, for example. This optical element is comprised of an electrooptic element in which prism-shape polarization inverted regions are formed in advance, to increase a difference in the refractive index in each prism region by applying a voltage and increase the deflection angle.
The direct electric field impression method is a known method for forming prism-shape polarization inverted regions in the electrooptic element. It is widely used in manufacturing a cyclic polarization inverted structure to generate a second harmonic from a nonliner optical crystal. The polarization inverted region is formed by applying a voltage with or over a coercive electric field between the top and bottom faces of an electro-optic substrate with electrodes formed in a desirable shape on the top and bottom faces. The mechanism of this polarization inversion is disclosed in detail in a publication, “Basics and Application of Polarization Inverted Device”, The Optronics Co., Ltd, for example.
Despite of its inexpensive price, high phase transition temperature, and stability at ambient temperature, niobate lithium used in the electrooptic element has a problem in optical damage resistance in a visual light range so that when light in a visual light range is guided thereto, the phase of the guided light is varied, causing the beam profile of emitted light to be distorted. In view of solving this problem, a magnesium-oxide-doped lithium niobate with high optical damage resistance in the visual light range has been developed.
The invertors of the present invention actually formed the prism-shape polarization inverted regions using the magnesium-oxide-doped lithium niobate as an electrooptic material and found out that it is difficult to accurately form the boundaries of polarization inverted regions. Specifically, while the prism-shape polarization inverted regions formed from the niobate lithium have linear interfaces and sharp apex angles, those formed from the magnesium-oxide-doped lithium niobate have curved optical incidence and exit faces 901, 902 on the interfaces and rounded apex angles 903 as shown in FIG. 13A. FIG. 13B shows a distorted profile of emitted light deflected through the polarization inverted regions in FIG. 13A. The distorted profile of emitted light leads to degradation of the performance of the optical deflector such as deterioration in the shape of emitted light or reduction in resolution.